1. Field of the Invention
The invention relates to a process for measuring the stress of a magnetic material, particularly, to an improvement in the process for measuring the stress of a magnetic material by utilizing a magneto-mechanical property of the magnetic material; an FRP (fiber-reinforced plastics) member whose internal damage is detectable, particularly, an FRP member whose internal damage can be detected by utilizing a magneto-mechanical property of a soft magnetic material; and an adhesive layer forming adhesive member whose internal defection is detectable, particularly, an adhesive member for forming an adhesive layer for bonding one member to another member, and for enabling detection of an internal defection thereof by utilizing a magneto-mechanical property of a soft magnetic material.
2. Description of the Prior Art
Such a stress-measuring process is conventionally known, which uses, as a measurement value, a variation in distribution of higher harmonic wave components including stress information concerning a magnetic material in an output signal, i.e., a ratio of outputs of two types of higher harmonic wave components (see Japanese Patent Application Laid-open No.201126/1986).
However, this prior art process has a problem in that the output ratio is varied such as to describe a curve having one peak with respect to a variation in stress. Therefore, notwithstanding that the stress of the magnetic material is different on opposite sides of a peak value, the same output ratio may be measured in some cases, resulting in inaccuracy.
An FRP member of the above-described type is known which includes a long carbon fiber embedded in an FOP member body (see Japanese Patent Application Laid-open No. 114741/1985).
Thus internal damage to this FRP member is detected by measuring an electric resistance of the long carbon fiber. This is based on a consideration that if an internal damage is produced in the FRP member, the long carbon fiber is broken, and the electric resistance value is varied.
However, this prior art FOP member has a problem in that it is impossible to detect a fine internal damage which is so small that it does not cause the breaking of the long carbon fiber, e.g., a hair crack produced in a plastics matrix surrounding the long carbon fiber, or an interfacial separation between the long carbon fiber and the plastics matrix.
Conventionally known methods for inspecting the adhesive layer include a performance test method according to the general rules of a weathering resistance test method defined in JIBS-K-6860, and a non-fracture inspecting method such as an ultrasonic damage searching method, and an X-ray method.
However, the performance test method is a method applied only to a test piece and hence it is very difficult to inspect an adhesive layer present between two bonded members. 0n the other hand, the non-fracture test method can provide an inspection result in its own way if it is applied to a test piece. However, the non-fracture test method is unsuitable for an adhesive layer of the above-described type, because there are many limitations to the shape, size and the like of the adhesive layer.